The simplest form of hot melt adhesive applicator has an open "glue pot" housing a wheel for effecting the application and an adjacent reservoir or "tank" maintaining the requisite level of adhesive in the glue pot, which along with the tank is heated electrically, to melt adhesive put in the tank and raise it to the required temperature in the glue pot. The applicator wheel is usually preceded by a doctor blade controlling the spread and thickness of adhesive, and is usually followed by an anti-stringout wheel contrarotating with respect to the applicator wheel so as to return to the glue pot any adhesive "strings" trailing from a flap that has passed over the applicator wheel. Although this form of hot melt adhesive applicator is highly suited to the accurate application of a wide variety of adhesives, it suffers from a number of disadvantages such as poor temperature control because of the volume in the glue pot, risk of degradation of the adhesive, contamination due to the open pot, and difficulty with replenishing the tank because it lies with the glue pot within the normally guarded regions of the packaging machinery. The glue pot can be provided with a cover (the tank invariably has one) but the need for openings for the applicator wheel and the anti-stringout wheel makes for difficulty in effecting adjustments to the position of the applicator wheel.
Remote supply systems are also known in a variety of forms.
Firstly, an enclosed wheel glue pot in which the sides of the pot fit closely to the sides of the applicator wheel and the adhesive is pumped from a remote tank through a flexible heated hose and passages in one side of the glue pot to a pool between the underside of the doctor blade and the periphery of the applicator wheel. The pump is controlled by sensing either the temperature or the pressure of the adhesive in the pool; in either case the control is complex and therefore liable to variation of the condition of the adhesive on the applicator wheel, and in the latter case high pressure flexible hose is required from the tank to the glue pot.
Secondly, jetting of the adhesive through a nozzle instead of application by a wheel in a glue pot. Although replenishment is easy and temperature control at the nozzle is good, this system is limited as to the types of adhesive which can be used, particularly because degradation can takeplace at the nozzle, and any contamination is likely to block the nozzle, but its particular disadvantage is that the nozzle must be controlled by a valve operated in response to a box or carton sensor, which is complex and very variable in accuracy.
Another system that has been tried but with limited success is one in which solid rod adhesive is mechanically fed to an enclosed wheel glue pot, the big disadvantage being that the heater in the glue pot has to effect melting of the adhesive and raise it to the required temperature at the wheel within a very short time and distance. This has proved practical with only one type of adhesive specifically formulated for the purpose; consequently, a number of installations have been converted to the remote supply type.